In a typical communications network, also referred to as a wireless communication network, communication system or wireless communication system, a User Equipment (UE), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operators network and services outside operator's network to which the operators radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised, or vehicle mounted devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly with the network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via or comprised in the radio access network and possibly one or more core networks and possibly the Internet.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g. a Radio Base Station (RBS), which in some radio access networks is also called evolved NodeB (eNB), NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. Thus, the communications network may also be referred to as a cellular network. The base stations communicate over the air interface operating with the user equipments within range of the base stations. The base station will be referred to as BS in some of the drawings.
A network deployment strategy which is more or less generally assumed to be dominant in future cellular networks is the so called heterogeneous networks. In a heterogeneous network large and small cells, high power and low power base stations/access points are mixed with each other in a largely overlapping fashion. Different cells may also employ different Radio Access Technologies (RATs), such as LTE FDD and LTE TDD, other RATs of the Third Generation Partnership Project (3GPP) family. Other RATS of the 3GPP family may be the Universal Mobile Telecommunications System (UMTS)/High Speed Packet Access (HSPA) and/or Global System for Mobile communication (GSM)/General packet radio service (GPRS)/Enhanced Data rates for GSM Evolution (EDGE), and non-3GPP RATs, e.g. IEEE 802.11x family (where “x” may be e.g. “a”, “b”, “g” or “n”) RATs or 3GPP2 RATs, e.g. Code division multiple access 2000 (CDMA2000). LTE is short for Long Term Evolution, FDD is short for Frequency Division Duplexing and TDD is short for Time Division Duplexing. IEEE is short for Institute of Electrical and Electronics Engineers. Other concepts that may be utilized to create the heterogeneous environment include e.g. Remote Radio Units (RRUs) and so called “baseband hotels”, where multiple baseband processing units are located at a semi central site. In a heterogeneous network the same location is often covered (i.e. within the radio transmission and reception coverage area) by more than one cell and base station or access point. Pico cells are often deployed to provide greater capacity in hotspot areas with dense user population and intense wireless communication, while macro cells provide the overall wide area coverage. Pico and femto cells, as well as RRUs may also be deployed to improve coverage in locations which are poorly covered by the macro cell layer, e.g. indoors.
Another topic that currently receives a lot of attention in the communications network industry is the importance of energy efficiency, both for the purpose of reducing OPerational EXpenses (OPEX) and in order to save the environment by reducing the CO2 footprint of communications networks. Another motive is to create goodwill for the operators and for the industry as a whole.
Combining heterogeneous networks and energy efficiency provides opportunities to leverage the greater number of base stations and the layered cell architecture of heterogeneous networks which often have overlapping coverage areas. To this end, schemes have been proposed where some cells of one layer, e.g. pico cells, are powered down (put in sleep mode) when the data traffic load and resource demands are low enough for the macro layer to handle. This principle may be used even in a non overlapping scenario, where instead the coverage areas of neighboring cells are extended, either by tilting up the antennas or utilizing reconfigurable antenna system techniques or adaptive antenna mechanisms, so that they cover the area of a cell that is currently in sleep mode.
Sleep mode may be utilized on various time scales and various levels in terms of affected hardware. The sleep mode may affect the entire base station, the part of the base station responsible for a certain cell, the transmission equipment, including a Power Amplifier (PA), or single circuit boards or components being part of pooled equipment or components. The time scale may be hours, e.g. shutting down an entire base station during nighttime, all the way down to milliseconds, e.g. putting single components, or even the PA, into sleep mode on an LTE subframe basis.
Such energy saving sleep mode strategies may be utilized even in deployments where the coverage is not taken over by a higher layer or neighboring cells. Potential scenarios may be low load periods (even empty cells which are not uncommon) or in combination with controlled Discontinuous Transmission (DTX) strategies where the users equipment(s) in the cell know when to expect and when not to expect transmissions, such as reference signals and system information, from the base station. On demand wakeup of sleeping cells to be ready to receive user equipments handed over from neighbor cells is another mechanism that may be used in conjunction with energy saving based on sleep mode.
A problem with the state of the art energy saving techniques in heterogeneous networks is that base stations often have embedded transport network transmission equipment, e.g. in the form of routers, switches (e.g. Ethernet switches) or SDH/PDH equipment, so that the connectivity of one base station depends on the transport network transmission equipment of another base station. SDH is short for Synchronous Digital Hierarchy and PDH is short for Plesiochronous Digital Hierarchy. An example is the commonly used tree deployment, wherein each branch comprises a number of cascaded base stations that depend on each other, as illustrated in FIG. 1. The right branch in FIG. 1 comprises four base stations A, B, C and D. The left branch in FIG. 1 comprises three base stations E, F and G. The gateway, referred to with GW in FIG. 1, is a switch or a router that concentrates the traffic to a link northbound from both branches. The fact that the connectivity of one base station depend on the transport network transmission equipment in one or more other base stations interfere with the sleep mode strategy, because the entire base station cannot be put in sleep mode even when all its cells are shut down, because the embedded transport network transmission equipment has to be active to provide connectivity to other base stations, e.g. the base stations further out on the same branch in a tree deployment (unless there are no active base stations further out on the branch). For example, base station B in the right branch in FIG. 1 cannot be put in complete sleep mode, unless base station C and D are also sleeping. The embedded transport network transmission equipment may represent a non negligible part of the energy consumption of a base station and thus represents a significant energy saving potential which unfortunately cannot be realized in many deployments.